Thanks for the responses guys. Math was never my strong point either, but I wanted to at least understand the method involved. One question still remains unanswered here though. If the saddle has undergone a few adjustments already and is as low as possible, and assuming I wanted to use the equation to calculate the amount of material removed from the heel, wouldn't I want to begin by re-establishing the proper break angle at the saddle with a new saddle? I think this question is getting passed over a bit only because the common practice (which is totally fine) for most would be to get the straightedge to read properly at the front ledge of the bridge, complete the reset, and then make a new saddle. While i'm at it, how is the amount of taper in the shims calculated when re-fitting the dovetail? This could totally be another topic unto itself. I get the carbon paper method for showing areas that keep it from fitting properly, but I guess I don't fully understand the process of shimming. Again, this looks like it could be more calculated (Nathan's method in his 2nd link above), or just trial fitment. What are these neck set feeler gauges mentioned?
Okay, let me expand a little.
The formula I use is explained in the Taylor reset article linked to above. It's actually two formulas.
The first determines how much the neck has moved as measured at the bridge. This formula works no matter how low or tall the saddle is or how high or low the action is.
The second formula tells you how much to remove at the heel cap.
Neck Set Feeler Gauges
I have clear plastic packaging lying around in a couple of different thicknesses. One is the "blister" packaging from a table saw blade etc… I cut one strip of plastic to the length and width of the shim then glue up layers of packaging on the bottom 8mm or so of the first piece of plastic.
I have a small drawer full of these of various thicknesses. I'll stick one of the "neck set feeler gauges" into the neck joint with the fat part of the feeler gauge at the bottom of the joint and dry fit the neck. I use trial and error until the neck is sitting about 3/32" above the top of the guitar by the neck joint. I'll divide that number by 2 and write it down. Then I'll do the same thing but with the fat part of the feeler gauge at the top of the joint. Now I know what the rough thickness of both shims at the bottom and top of the joint are. I'll make the shims to those specs (maybe adding .010" of overall thickness or whatever if there is a low spot from the factory in the neck block or tenon) then glue them in with my little neck set go bars.
Dental Articulating Paper
Then it's a matter of dry fitting the joint with a piece of dental articulating paper on each side of the joint. The articulating paper is better than carbon paper because 1) it's easier to see 2) it marks both the tenon and the mortise. I'll then trim away the blue marks from all four surfaces and repeat the process until the neck is just about seated. *Since I'm working all four surfaces at once this process is very fast. At that point, I will use one piece of articulating paper and just trim one surface until the joint is just about right. My last bit of chisel work is paring away the blue from both surfaces of that one side of the joint. Some acetone cleans up any remnants of the blue articulating paper from the neck block and tenon.
I rather sorry that I posted that link now that I actually spent a bit of time looking at the method. It seems to be a rather round about way of getting a value for what is actually just a change in the height of saddle protrusion. Uh, yes, the low saddle height is being subtracted from a projected or ideal saddle height but you could just measure what the saddle height is now, subtract it from what you want it to be and divide by two for what the result would be at the 12th fret.
There are different approaches to shimming. I make shims and glue one to either side of the dovetail. I figure an approximate thickness for the shims by assembling the neck to the neck pocket with folder paper on both sides of the dovetail. When I have enough folded layers of paper that the neck heel is pulled down flat to the body, I can't wiggle the neck side to side or forward and the neck won't go all the way in, I take it apart and measure the paper thickness. The shims are cut to that or slightly thicker and glued in place.
When you remove material from the neck heel, the neck tips back and the wedge shape of the dovetail drops away from the beveled neck pocket. Installing and shaping the shims adds back the wood necessary to mate back up with the neck pocket. I like chalk for dry fitting to see where material should be removed. With a small sanding block, I just sand the spots with chalk on them. This is done trial and error, over and over and over until the the fingerboard will just about be seated down onto the top. You must pay attention to neck alignment as you go so both sides of the finger board are the same dimension from the top at all times. Other alignments are dealt with when the heel sanding is done.
You really can't pre-calculate the shims other than a rough initial starting thickness. It's pretty much trial and error the whole way. The feeler gauges Nathan mentions are just stacked to a thickness equal to what needs to be removed at the neck heel as a way to scratch or mark an accurate line on the heel cap.
The feeler gauges Nathan mentions are just stacked to a thickness equal to what needs to be removed at the neck heel as a way to scratch or mark an accurate line on the heel cap.
I stand corrected on this Nathan, is this what your doing with Taylor guitars? I don't understand how your home made feeler gauges help with a dovetail neck re-set.
Check out photos 12 and 13 here (read the captions too):
Thanks Nathan, I have to admit looking at your first link but not the other where this was explained.
To answer the question on making the shims. No math involved here either. I just make a rough guess on how thick to make the shims, by how loose the neck fit is before shimming. I then take the two shims( 1 for bass side 1 for treble side) using double sided tape I tape them to a block of wood. Now I thin and taper shims with a belt sander. When I think they are getting close to fitting, I remove from the block and try the fit. will keep sanding until the fit is almost perfect. Then I clamp shims and neck in joint. Now remove neck and see where there are shinny or rub marks on the shims, I will sand these areas down more. Do this until neck fits solid in the joint and is fully seated.
Now the neck is ready to glue in.
An observation I normally take before doing a neck reset is where the straightedge hits the bridge with guitar at pitch and where it hits the bridge without string tension. This is how much the body flexes and the top rises. When the neck is reset I want the straightedge to hit the very top of the bridge once strung to pitch. So when doing the reset I may shoot for the straightedge being 1/32" to 1/16"(at the most) above the bridge.
It's impossible to have the same saddle height on every instrument as they all don't flex the same. Then take into account variable environments that instruments are subject to over time, different gauge strings ect.